ConfHandler.py: Add a hook for config parsing

To make the UI settings take effect, we need to hook at the end of each
config file parsing and set UI specific values.

(Bitbake rev: f54e733c7863110896f43900d9e4e791602f9d65)

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
Signed-off-by: Dongxiao Xu <dongxiao.xu@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

bitbake/lib/bb/command.py

@@ -179,6 +179,13 @@ class CommandsSync:
         """
         return bb.utils.cpu_count()
 
+    def setConfFilter(self, command, params):
+        """
+        Set the configuration file parsing filter
+        """
+        filterfunc = params[0]
+        bb.parse.parse_py.ConfHandler.confFilters.append(filterfunc)
+
 class CommandsAsync:
     """
     A class of asynchronous commands

bitbake/lib/bb/parse/parse_py/ConfHandler.py

@@ -71,6 +71,14 @@ def include(oldfn, fn, lineno, data, error_out):
             raise ParseError("Could not %(error_out)s file %(fn)s" % vars(), oldfn, lineno)
         logger.debug(2, "CONF file '%s' not found", fn)
 
+# We have an issue where a UI might want to enforce particular settings such as
+# an empty DISTRO variable. If configuration files do something like assigning
+# a weak default, it turns out to be very difficult to filter out these changes,
+# particularly when the weak default might appear half way though parsing a chain 
+# of configuration files. We therefore let the UIs hook into configuration file
+# parsing. This turns out to be a hard problem to solve any other way.
+confFilters = []
+
 def handle(fn, data, include):
     init(data)
 
@@ -107,6 +115,9 @@ def handle(fn, data, include):
     if oldfile:
         data.setVar('FILE', oldfile)
 
+    for f in confFilters:
+        f(fn, data)
+
     return data
 
 def feeder(lineno, s, fn, statements):